HH135/HH136 - a luminous H$_2$ outflow towards a high-mass protostar

Near-infrared observations towards the luminous IRAS source IRAS11101-5928 and the associated Herbig-Haro objects HH135/HH136 are presented. The observations reveal the presence of a well-collimated, parsec-sized H$_2$ outflow with a total H$_2$ luminosity of about $2L_\odot$. The bulk of the molecular gas is characterized by a ro-vibrational excitation temperature of $2000\pm200$ K. A small fraction (0.3%) of the molecular gas is very hot, with excitation temperatures around 5500 K. The molecular emission is associated with strong [FeII] emission. The H$_2$ and [FeII] emission characteristics indicate the presence of fast, dissociative J-shocks at speeds of $v_\mathrm{s} \approx$ 100 km s$^{-1}$. Electron densities of $n_\mathrm{e}$ = 3500-4000 cm$^{-3}$ are inferred from the [FeII] line ratios. The large H$_2$ luminosity combined with the very large source luminosity suggests that the high-mass protostar that powers the HH135/HH136 flow forms via accretion, but with a significantly increased accretion rate compared to that of low-mass protostars.